The immune system protects the body from invading disease-causing organisms, or pathogens. Pathogens and other non-self molecules are antigens – foreign molecules recognized by the immune system, stimulating an immune response. The majority of infections by pathogens occurs in mucous membranes of our body.

Innate defenses act immediately or within hours of a pathogen's appearance in the body. Innate defenses are nonspecific – they target any pathogen. Innate defenses include:

• Skin, which excludes most pathogens from entering the body.
• Cilia in mucous membranes, which sweep out airborne pathogens and dust.
• Tears, nasal secretions and saliva, which contain bacteria-destroying enzymes.
• Phagocytic cells, called neutrophils, macrophages, and dendritic cells.

Phagocytes ("phago-"=eating, "cyte"=cell) migrate to affected areas and engulf pathogens. Neutrophils and macrophages are phagocytic white blood cells. This migration of white blood cells causes the redness and inflammation associated with infection. Some cells of innate immunity are of special importance for regulating our immune response. These cells called dendritic cells or Langerhans cells can move through out our body, and are particularly rich in our skin and mucus membranes of our body that are exposed to foreign material, including our disgestive systems, airways, and sexual apparatuses. When dendritic cells encounter foreign material, they also are phagocytic (eat the material), but have special receptors that allow them to distinguish harmless and pathogenic (disease causing) organisms. However, these cells carry fragments of pathogen to lymph nodes where they either prevent or stimulate an adaptive immune response. The decision about which response to cause depends on the foreign material (dangerous pathogens cause a dramatic response) and whether cells of your own body are sending out "danger" or distress signals. The significance of the dendritic cells is that they can prevent you from reacting against your own tissues, against food that you ingest or harmless materials from your environment, or they can tell the rest of your immune system to make an adaptive immune response.

Microscopic movie of macrophages ingesting a yeast (567 kb)

If innate immune cells (dendritic cells) decide that the material is dangerous (part of a virus or bacteria), then they stimulate a specialized group of white blood cells causes CD4+ helper T cells to become activated. CD4+ refers to a surface protein on this class of T cells. Helper T cells can stimulate another group of white blood cells called B cells to produce antibodies that bind that specific antigen and immobilize it, preventing it from causing infection. Antibodies are specific for only one antigen. B cells must interact with Helper T cells, other specialized white blood cells, to initiate antibody production. An important concept is that once activated, memory cells are produced that insure that a more rapid and stronger immune response can be made upon re-exposure to the same pathogen. This is why vaccinations provide lasting protection against disease. Memory helper T cells are labeled CD4+ CCR5+, to note that the chemokine receptor (CCR5) is present on the surface of the helper T cell. These cells migrate and reside in the mucus membranes of our body.

Pathogens (viruses or bacteria) that escape antibody detection can enter and infect cells. The surface of infected cells changes, and this change is recognized by T cells. Cytotoxic T cells kill infected cells, preventing these cells from producing more pathogen. Cytotoxic T cells must interact with Helper T cells to regulate destruction of infected cells. Remember that the dendritic cells must activate CD4+ helper T cells before our bodies can produce B cells secreting pathogen specific antibodies or cytotoxic T cells to destroy infected cells.)

Human Immunodeficiency Virus (HIV) specifically attacks Helper T cells. Without an adequate supply of Helper T cells, the immune system cannot signal B cells to produce antibodies or Cytotoxic T cells to kill infected cells. When HIV has critically depleted the Helper T cell population, the body can no longer launch a specific immune response and becomes susceptible to many opportunistic infections. This immunodeficiency is described in the name acquired immunodeficiency syndrome, or AIDS. Recent work shows that the CD4 and CCR5 membrane proteins are targets for HIV infection. Thus, our memory cells are rapidly infected and destroyed in the mucus membranes of our tissues. We have only recently recognized that the memory cell destruction occurs in the first several days after HIV infection, suggesting that therapies should begin as soon as the infection is recognized.